Avian brood parasites are virulent, both as adults and as nestlings, because they reduce the fecundity of their hosts. The extent of virulence varies widely, both within and between brood parasite species. Here I review previous explanations for variation in the harm that brood parasites inflict on their hosts, which focus largely on the benefits of virulence, and suggest that each hypothesis is in some way unsatisfactory. I then summarize the evidence that brood parasitic offspring experience costs when host young die. I argue that the virulent behaviours shown by brood parasites are exactly analogous to the virulence shown by pathogens. Both can experience benefits by damaging host fitness, but they come at a price. I suggest that the trade-off hypothesis, developed with some success for understanding the evolution of virulence in pathogens, ought to be adopted in future theoretical and empirical work on the evolution of virulence in brood parasites.
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